fix(desktop): let the edit composer scroll long prompts

[mollusk]

Symptom

When you click a previous prompt in Hermes Desktop, the bubble briefly expands to show the full text, then snaps back to a clipped view — and you can't scroll within the editor, so the tail of a long prompt becomes unreachable.

Root cause

Clicking a read-only user bubble opens the inline edit composer. Its contentEditable editor (apps/desktop/src/components/assistant-ui/thread.tsx) caps its height at max-h-48 but had no overflow rule, and its container is overflow-hidden. So a prompt taller than the cap is clipped with no scrollbar.

The "expands then shrinks" flash is the read-only bubble lifting its 2-line sticky-human-clamp on focus right before the edit composer mounts and clips.

The main chat composer's editor (apps/desktop/src/app/chat/composer/index.tsx) already pairs its height cap with overflow-y-auto; the edit composer simply missed it.

Fix

Add overflow-y-auto to the edit composer editor — one class, matching the main composer. Overflow now scrolls within the capped height instead of being hidden.

Test

Added a regression to user-message-edit.test.tsx asserting the edit composer editor keeps both the height cap (max-h-48) and a scroll affordance (overflow-y-auto). Confirmed RED before the fix, GREEN after.

Verification

vitest run --environment jsdom src/components/assistant-ui/user-message-edit.test.tsx   # 3 passed
eslint src/components/assistant-ui/thread.tsx src/components/assistant-ui/user-message-edit.test.tsx   # clean
tsc -b   # clean

[teknium1]

Thanks for the focused desktop fix. Current main still renders the inline edit contentEditable with max-h-48 but no overflow behavior at apps/desktop/src/components/assistant-ui/thread/user-edit-composer.tsx:632; the main composer already uses overflow-y-auto for its capped editor at apps/desktop/src/app/chat/composer/index.tsx:717.

No substantive problems identified. The PR's one-class change and focused regression assertion match the confirmed failure path. The target files are unchanged from the PR base on current main, so this should be mechanically salvageable.

Automated hermes-sweeper review.

[teknium1]

Thanks for this fix! It was salvaged into #86766 (cherry-picked onto current main with your authorship preserved in the commit history) and is now merged. Closing since the work has landed.